I found out that I was pregnant with #2 when my son was 14 months old. I weaned him over the next week. (He was ready and thankfully was sleeping all night.) I was also working and I knew that my body couldn't take nursing and being pregnant.
My kids are now 7 1/2 and 5 1/2+. (They're 21 months apart, she came early.) Your son will never really remember life without his younger sibling, and I found that adding the new baby was really not a big issue since he was so young...I cuddled and quietly played with him the same way I had when I was pregnant. My 2 are very close and have played together since my daughter was old enough to play. They entertain each other and I don't think it would have been nearly as easy to parent them in their preschool years if I had had to entertain them each on their own. (I have a friend whose kids are 4 years apart, her oldest is the same age as my youngest. Her oldest was very needy and always wanted to play with Mommy, Mommy, Mommy. My 2 like to play with Mommy but also like to play with each other...probably more than with Mommy.
Congruatulations and good luck! You have a lot on your plate right now!